1. Field of the Invention
The present invention relates to a support for photographic use, and more particularly, to a polyester film of such type which is adapted to X-ray photographic film supports.
2. Description of the Prior Art
Polyester films used as supports for X-ray photographic films are usually colored to provide a blue or bluish purple color in order to render the image on the photographic film easily discernible and to minimize the eye strain of the viewer observing the image on a viewing lantern.
Ordinary blue or bluish purple films have a spectrum of shades ranging from red to green and may be light or dark. However, for the reasons peculiar to supports for use in X-ray photography, the films designed for use as such supports are required to have a very limited range of color tones. Japanese Patent Publication No. 8734/1972 disclosed the preferred chromaticity range for polyethylene terephthalate films used as supports for X-ray photographic films.
As evidenced by British Pat. No. 1,137,595, Belgian Pat. No. 715,614 and Japanese Patent Publication No. 33724/1976, the conventional efforts made to obtain the desired blue or bluish purple color in polyester films for X-ray photographic film supports have centered around the screening of colorants. On the other hand, because of the extremely stable physical structure of polyester films, it is difficult to color them uniformly by such techniques as solvent coloring and surface coating. Instead, bulk coloring techniques which are to incorporate organic coloring agents into the base are more commonly employed, wherein a colorant is dispersed or dissolved in the polyester being synthesized or incorporated into the polymer being melt-formed. If one or more colorants alone are introduced into the polyester, variations in color shade occur unavoidably while the polymer is processed into a film under the conventional polyester film forming conditions. Yellowish films shades are particularly objectionable because they render the photographic image less easily discernible and cause increased straining of the viewer's eye.
Unexamined Published Japanese Patent Application No. 734/1980 discloses a Color Index Solvent Blue 97 as a colorant that suffers a smaller variation in color at any stage of the film production, but even this colorant is not completely effective in solving the problems associated with polyester films intended for use as supports for X-ray photographic films.
The present inventors made various studies to attain the purpose of producing a colored polyester film for X-ray photographic film supports having a stabilized color tone without experiencing any variation in color at any stage of the film production. As a result, the inventors have found that if not only a colorant but also a quaternary ammonium salt is incorporated in a polyester, a film having the desired color can consistently be produced without experiencing any change in color during the process of film production.